This invention relates to a method of brazing a porous material to a background material, and more specifically, this invention relates to a method in which a braze flow barrier is provided which defines a preselected area of the porous material in order to contain and limit braze flow during the formation of the braze joint.
Research and development of rechargeable liquid molten salt batteries has been progressing on a large scale for several years. In these batteries, a molten electrolyte such as a eutectic combination of lithium chloride and potassium chloride having a melting point of 352.degree. C. is employed between several parallel-connected anodes and cathodes. Electrical insulation between the electrodes is provided by porous ceramic separators which permit ionic conduction through the molten eutectic potassium chloride and lithium chloride electrolyte. Because of the corrosive nature of the electrolytes and the operating temperatures necessitated, such as 400.degree. C. or above, substantial developments has been necessary to provide corrosion resistant materials from which to construct the negative anode electrodes and the positive cathode electrodes as well as the separators and therebetween.
A rechargeable lithium-silicon/iron sulfide load-leveling battery which utilizes a lithium chloride/potassium chloride eutectic electrolyte provides a very attractive combination due to its potential long life. Development of a corrosion resistant positive cathode electrode for the iron sulfide has been a primary objective. A honeycomb structure which is useful to prevent swelling, slumping and extrusion of the active iron sulfide material during battery operation has been described in U.S. Pat. No. 4,003,753 issued Jan 18, 1977 to John C. Hall for Electrode Structure For Electrical Energy Storage Device, the disclosure of which is incorporated herein by reference. This patent discloses a honeycomb structure for storing the positive cathode electrode active material connected to a porous membrane which acts as a barrier between the positive cathode electrode active material and the molten lithium chloride/potassium chloride eutectic electrolyte. For proper operation of the battery, it is essential that the porous nature of the membrane remain intact, thereby to permit the beforementioned ionic conduction through the electrolyte between the electrodes.
Various metals are acceptable for use as porous membranes, the membranes generally being thin, having a thickness less than one millimeter thick and preferrably having a thickness of about 0.25 millimeters. Porosity and pore size of the membrane may vary considerably; however, an acceptable membrane may have a porosity in the order of about 40% with a pore size of about 2 microns. Available and acceptable materials for the porous membrane is any material with the requisite porosity, corrosion resistance and melting point which can be suitably connected to the associated electrode and include nickel, the Hastalloys, the Inconels, other nickel-iron alloys, various stainless steels and various carbon steels.
Because of the nature of the liquid metal batteries, it is necessary to have a thermodynamically acceptable material connecting or bonding the thin porous membrane to the positive cathode electrode structure, of honeycomb in this particular battery application. Because of the complex physical structure of the honeycomb, brazing is particularly attractive as opposed to other methods of connecting or bonding the membrane and the electrode structure. A potential braze material, which the thermodynamic considerations indicate should have an acceptable corrosion resistance is nickel phosphide. The nickel-phosphorous phase diagram discloses a eutectic with a melting point of 880.degree. C. and a composition of 89 weight percent nickel and 11 weight percent phosphorous. An alloy of this composition or one having a higher phosphorous content may be electrolessly deposited on various substrates according to the method disclosed in the Guitzeit et al. U.S. Pat. No. 2,690,401, the Talmey et al. U.S. Pat. No. 3,325,297 and the Parker et al. U.S. Pat. No. 3,887,732, the disclosure of the last named patent being incorporated herein by reference.
The problem incurred in attempting to form a braze joint between a porous membrane and a substrate is that at the elevated temperatures necessary to form the brazed joint, the braze material wicks or flows through the porous membrane, thereby substantially sealing the pores and rendering the membrane ineffective for use as a structural member in the positive cathode electrode in a liquid metal-molten salt battery which requires ionic conduction through the porous structural member.